1. Field of the Invention
This invention relates to a blood vessel imaging system for imaging blood vessels, and more particularly to a blood vessel imaging system which can image blood vessels with arteries and veins distinguished from each other.
This invention further relates to a system which can clearly distinguish arteries and veins from each other.
This invention further relates to a system for measuring the flow rate of light scattering fluid such as arterial blood and venous blood.
2. Description of the Related Art
In the clinical field, there has been a wide demand for imaging arteries and veins clearly distinguished from each other. For example, since arteriosclerosis generally starts at the periphery of the arteries, it will be useful in diagnosing arteriosclerosis if the inner walls of the peripheral arteries can be imaged distinguished from those of the veins.
There has been wide known angiography as a system for imaging blood vessels. However angiography is disadvantageous in that load on the testee is heavy and the testee generally must stay in the hospital.
Further there has been proposed technique for imaging part of an organism on the basis of penetration of light through the part as disclosed in xe2x80x9cJapanese ME Academy Magazine BMExe2x80x9d, vol.8, No.5, 1994, pp.41xcx9c50. However it is very difficult to image arteries and veins clearly distinguished from each other by the technique.
In view of the foregoing observations and description, the primary object of the present invention is to provide a blood vessel imaging system which can image blood vessels with arteries and veins clearly distinguished from each other without exposing the testee to heavy load.
Another object of the present invention is to provide a blood vessel distinguishing system which cyan clearly distinguish arteries and veins from each other without 9 sing the testee to heavy load.
Still another object of the present invention is to provide a flow rate measuring system for measuring the flow rate of light scattering fluid such as arterial blood and venous blood. In the blood vessel imaging system in accordance with the present invention, an optical heterodyne detection system is employed in order to ensure high spatial resolution to an organism as a scattering medium, and arteries and veins are distinguished from each other on the basis of the fact that the spectral broadening (Doppler broadening) of beat component detection signal output from the heterodyne detection system changes with the flow rate of blood in the blood vessel.
That is, in accordance with a first aspect of the present invention, there is provided a blood vessel imaging system comprising
a measuring light source which emits a measuring light beam,
a scanning means which causes the measuring light beam to scan an organism,
an optical heterodyne detection system consisting,;of an optical system which splits the measuring light beam upstream of the organism into a first light beam traveling to impinge upon the organism and a second light beam traveling not to impinge upon the organism and combines the second light beam with the first beam emanating from the organism into a combined light beam, a frequency shifter which causes the first and second light beams to have frequencies different from each 0 and a beat component detecting means which detects beat components of the combined light beam,
a filtering means which detects, out of the beat component detection signal output from the beat component detecting means, off-centered components in a frequency band deviated from the center frequency of the beat component detection signal by a predetermined width, and
an image signal generating means which generates an image signal according to whether the off-centered beat signal detected by the filtering means is higher or lower than a predetermined threshold level.
For example, the image signal generating means generates an image signal representing artery parts of the organism on the basis of components of the off-centered beat signal which are higher than the predetermined threshold level, and generates an image signal representing vein parts of the organism on the basis of components of the off-centered beat signal which are lower than the predetermined threshold level.
It is preferred that the blood vessel imaging system be provided with an in-phase time detecting means which detects in-phase times at which broadening of the spectrum of the beat component detection signal becomes of a predetermined phase, and a synchronization detecting means which samples the off-centered beat signal at the in-phase times and inputs the off-centered beat signal thus obtained into the image signal generating means.
The in-phase time detecting means may be, for instance, a means for detecting the pulse wave of the organism, or for detecting the times at which the center frequency component of the beat component detection signal takes a predetermined peak value.
Further it is preferred that the measuring light source comprises a linear or two-dimensional array of a plurality of light emitting portions and the optical heterodyne detection system is arranged to be able to detect in parallel beat components of the combined light beams based on the measuring light beams from the respective light emitting portions, and the measuring light source and the optical heterodyne detection system also function as at least a part of said scanning means.
The beat component detection signal (beat signal) output from the heterodyne detection system described above represents intensity of only straight light components traveling straight through the organism or scattered light components close to the straight light components except influence of scattering by the organism which is a scattering medium.
When a fluid which causes multiple scattering of the measuring light flows in a direction perpendicular to the direction of travel of the measuring light, the peak value of the beat signal is lowered and the spectrum of the beat signal is broadened. For example, FIG. 4A shows a spectrum of the beat signal when the flow rate of the fluid is 0, and FIGS. 4B to 4D show those for different flow rates of the fluid which increase in this order. As can be seen from FIGS. 4A to 4D, the peak value of the intensity of the beat signal becomes lower and the spectrum of the beat signal is broadened (Doppler broadening) as the flow rate of the fluid increases.
Since blood is also a fluid which causes multiple scattering of light, the same phenomenon occurs when the measuring light beam passes through a blood vessel. Since arterial blood is generally higher than venous blood in flow rate, the reduction in the peak value of the intensity of beat signal and broadening of the spectrum are larger when the measuring light beam travels through an artery than when the measuring light beam travels through a vein.
In FIG. 5, line a shows a spectrum of the beat signal when the measuring light beam travels through an artery and line b shows a spectrum of the beat signal when the measuring light beam travels through a vein. When components of the beat signal in a frequency band deviated from the center frequency xcfx89 of the beat signal by a predetermined width xcex94f are detected by use of a band-pass filter having transmission characteristics shown by line c, and an image signal is generated on the basis of components of the off-centered beat signal (made up of components of the beat signal in said frequency band) which are higher or lower than a predetermined threshold level, an image signal representing only artery parts or vein parts of the organism can be generated.
That is, when the predetermined threshold value is set, for instance, at d in FIG. 5, and an image signal is generated on the basis of components of the off-centered beat signal which are higher than the predetermined threshold level d, a signal representing only artery parts of the organism can be generated. On the other hand, when an image signal is generated on the basis of components of the off-centered beat signal which are lower than the predetermined threshold level d, an image signal representing only vein parts of the organism can be generated.
More strictly, broadening of the spectrum of the beat signal due to flow of arterial blood changes with the flow rate of arterial blood. That is, the spectrum of the beat signal at a maximum flow rate of the arterial blood is as shown by line a-1 in FIG. 6 whereas the spectrum of the beat signal at a minimum flow rate of the arterial blood is as shown by line a-2. Broadening of the spectrum of the beat signal is very similar to that of beat signal when the measuring light beam travels through a vein (shown by line b) in a frequency band corresponding to the transmission frequency band of the band-pass filter, and accordingly it is difficult to distinguish the former from the latter.
Accordingly, when in-phase times at which broadening of the spectrum of the beat component detection signal becomes of a predetermined phase (optimally times at which the flow rate of the arterial blood is maximized) are detected, off-centered beat signal is sampled at the in-phase times and the off-centered beat signal thus obtained is input into the image signal generating means, arteries and veins can be imaged clearly distinguished from each other.
Further when a measuring light source comprising a linear or two-dimensional array of a plurality of light emitting portions and an optical heterodyne detection system which can detect in parallel beat components of the combined light beams based on the measuring light beams from the respective light emitting portions are employed so that the measuring light source and the optical heterodyne detection system also function as at least a part of said scanning means, it becomes unnecessary for the scanning means to mechanically cause the measuring light beam to scan the organism in at least one direction, whereby the scanning speed, which results in the imaging speed, can be increased. This is especially advantageous in the case where the off-centered beat signal is sampled at said in-phase times and sampling of the off-centered beat signal requires a relatively long time.
In accordance with a second aspect of the present invention, there is provided a blood vessel imaging system comprising
a measuring light source which emits a measuring light beam,
a scanning means which causes the measuring light beam to scan an organism,
an optical heterodyne detection system consisting of an optical system which splits the measuring light beam upstream of the organism into a first light beam traveling to impinge upon the organism and a second light beam traveling not to impinge upon the organism and combines the second light beam with the first beam emanating from the organism combined light beam, a frequency shifter which causes the first and second light beams to have frequencies different from other, and a beat component detecting means which detects beat components of the combined light beam,
a first intensity detecting means which detects the intensity of center frequency components of the beat component detection signal output from the beat component detecting means,
a second intensity detecting means which detects the intensity of off-centered components of the beat component detection signal in a frequency band deviated from the center frequency of the beat component detection signal by a predetermined width, and
an image signal generating means which generates an image signal on the basis of the ratio of the intensity of the off-centered components of the beat component detection signal to the intensity of the center frequency components of the beat component detection signal.
Preferably, the second intensity detecting means detects the intensities of first and second off-centered components of the beat component detection signal in different frequency bands, and the image signal generating means generates an image signal representing artery parts of the organism on the basis of the ratio of the intensity of the center frequency components to that of the first off-centered components and generates an image signal representing vein parts of the organism on the basis of the ratio of the intensity of the center frequency components to that of the second off-centered components.
In accordance with a third aspect of the present invention, there is provided a blood vessel imaging system comprising
a measuring light source which emits a measuring light beam,
a scanning means which causes the measuring light beam to scan an organism,
an optical heterodyne detection system consisting of an optical system which splits the measuring light beam upstream of the organism into a first light beam traveling to impinge upon the organism and a second light beam traveling not to impinge upon the organism and combines the second light beam with the first beam emanating from the organism into a combined light beam, a frequency shifter which causes the first and second light beams to have frequencies different from each other, and a beat component detecting means which detects beat components of the combined light beam,
a spectrum analysis means which obtains the spectrum of the beat component detection signal output from the beat component detecting means, and
an image signal generating means which generates an image signal on the basis of a spectral width between two frequencies at which the intensity of the beat component detection signal takes a predetermined value with respect to the intensity of the center frequency components, for instance, a half-width of the spectrum.
As described above, the beat component detect-ion signal (beat signal) output from the heterodyne detection system described above represents intensity of only straight light components traveling straight through the organism or scattered light components close to the straight light components except influence of scattering by the organism which is a scattering medium.
When a fluid which causes multiple scattering of the measuring light flows in a direction perpendicular to the direction of travel of the measuring light, the peak value of the beat signal is lowered and the spectrum of the beat signal is broadened. For example, FIG. 11A shows a spectrum of the beat signal when the flow rate of the fluid is 0, and FIGS. 11B to 11D show those for different flow rates of the fluid which increase in this order. As can be seen from FIGS. 11A to 11D, the peak value of the intensity of the beat signal becomes lower and the spectrum of the beat signal is broadened (Doppler broadening) as the flow rate of the fluid increases.
The ratio I(xcfx89+xcex94f)/I(xcfx89), to the intensity I(xcfx89) of the center frequency components of the beat signal (the components of the beat signal at the center frequency xcfx89), of the intensity I(xcfx89+xcex94f) of the off-centered components of the beat signal in a frequency band deviated from the center frequency xcfx89 by a predetermined width changes with the flow rate v of the fluid substantially as shown in FIG. 12.
Since blood is also a fluid which causes multiple scattering of light, the same phenomenon occurs when the measuring light beam passes through a blood vessel since arterial blood is generally higher than venous blood in flow rate, the ratio I(xcfx89+xcex94f)/I(xcfx89) is larger when the measuring light beam travels through an artery than when the measuring light beam travels through a vein.
Though the intensity I of the beat signal itself fluctuates affected by attenuation due to absorption and/or scattering when the measuring light beam travels through the organism, the intensity ratio, that is, the value obtained by normalizing the intensity I(xcfx89+xcex94f) of the off-centered components of the beat signal by the intensity I(xcfx89) of the center frequency components of the beat signal, changes basically depending solely on the flow rate of blood in the manner described above with the influence of the attenuation compensated for.
Accordingly, when the image signal generating means generates an image signal on the basis of the intensity ratio I(xcfx89+xcex94f)/I(xcfx89), for instance, so that the image signal takes a higher value as the intensity increases, the artery parts and the vein parts can be imaged to be clearly distinguishable from each other from density and/or brightness.
Since the intensity ratio I(xcfx89+xcex94f)/I(xcfx89) corresponds to the spectral waveform of the beat signal, the artery parts and the vein parts can be imaged to be clearly distinguishable from each other also by generating an image signal on the basis of the spectral waveform of the beat signal in place of the intensity ratio I(xcfx89+xcex94f)/I(xcfx89) as in the blood vessel imaging system of the third aspect.
That is, in the blood vessel imaging system in accordance with the third aspect of the present invention, the image signal is generated on the basis of a spectral width between two frequencies at which the intensity of the beat component detection signal takes a predetermined value with respect to the intensity of the center frequency components, for instance, a half-width of the spectrum. Since such a spectral width becomes larger as the flow rate of blood increases as can be seen from FIGS. 11A to 11D, the artery parts and the vein parts can be imaged to be clearly distinguishable from each other from density and/or brightness when the image signal is generated, for instance, so that the image signal takes a higher value as the spectral width becomes larger.
Further, since also the spectral width such as the half-width changes basically depending solely on the flow rate of blood in the manner described above with the influence of the attenuation compensated for, the artery parts and parts can be accurately distinguished from each other on basis of the spectral width.
In accordance with a fourth aspect of the present invention, there is provided a blood vessel distinguishing system comprising
a measuring light projecting means which projects measuring light onto an organism, and
an imaging means which images an artery and/or a vein in the organism on the basis of broadening of a spectrum due to an interaction of the measuring light with the organism.
It is preferred that the imaging means detects frequency components of the measuring light scattered by the organism, detects the half-width of the spectrum of the frequency detection signal and images an artery and/or a vein in the organism on the basis of the half-width.
The imaging means may be a means which detects frequency components of the measuring light scattered by the organism, detects the intensity of off-centered components in a frequency band deviated from the center frequency of the frequency detection signal by a predetermined width and images an artery and/or a vein in the organism on the basis of the intensity.
Further the imaging means may be a means which detects frequency components of the measuring light scattered by the organism, detects the ratio between the intensity of center frequency components of the frequency detection signal and the intensity of off-centered components in a frequency band deviated from the center frequency of the frequency detection signal by a predetermined width and images an artery and vein in the organism on the basis of the intensity ratio.
Further the imaging means may be a means which detects frequency components of the measuring light scattered by the organism on the basis of an optical heterodyne detection signal.
In accordance with a fifth aspect of the present invention, there is provided a blood vessel distinguishing system comprising
a measuring light projecting means which projects measuring light onto an organism, and
a distinguishing means which distinguishes an artery and a vein in the organism from each other on the basis of broadening of a spectrum due to an interaction of the measuring light with the organism.
It is preferred that the distinguishing means detects frequency components of the measuring light scattered by the organism, detects the half-width of the spectrum of the frequency detection signal and distinguishes an artery end a vein in the organism from each other on the basis of the half-width.
The distinguishing means may be a means which detects frequency components of the measuring light scattered by the organism, detects the intensity of off-centered components in a frequency band deviated from the center frequency of the frequency detection signal by a predetermined width and distinguishes an artery and a vein in the organism from each other on the basis of the intensity.
Further the distinguishing means may be a means which detects frequency components of the measuring-light scattered by the organism, detects the ratio between the intensity of center frequency components of the frequency detection signal and the intensity of off-centered components in a frequency band deviated from the center frequency of the frequency detection signal by a predetermined width and distinguishes an artery and a vein in the organism from each other on the basis of the intensity ratio.
Further the distinguishing means may be a means which detects frequency components of the measuring light scattered by the organism on the basis of an optical heterodyne detection signal.
In accordance with a sixth aspect of the present invention, there is provided a flow rate measuring system for measuring a flow rate of light scattering fluid comprising
a measuring light projecting means which projects measuring light onto light scattering fluid, and
an analysis means which analyzes the flow rate of the light scattering fluid on the basis of broadening of a spectrum due to an interaction of the measuring light with the light scattering fluid.
It is preferred that the analysis means analyzes frequency components of the measuring light scattered by the organism, detects the half-width of the spectrum of the frequency detection signal and analyzes the flow rate of the light scattering fluid on the basis of the half-width.
The analysis means may be a means which detects frequency components of the measuring light scattered by the organism, detects the intensity of off-centered components in a frequency band deviated from the center frequency of the frequency detection signal by a predetermined width and analyzes the flow rate of the light scattering fluid on the basis of the intensity.
Further the analysis means may be a means detects frequency components of the measuring light scatter by the organism, detects the ratio between the intensity of center frequency components of the frequency detection signal and the intensity of off-centered components in a frequency band deviated from the center frequency of the frequency detection signal by a predetermined width and analyzes the flow rate of the light scattering fluid on the basis of the intensity ratio.
Further the analysis means may be a means which detects frequency components of the measuring light scattered by the organism on the basis of an optical heterodyne detection signal.
In the blood vessel imaging system in accordance with the fourth aspect of the present invention, an artery and/or a vein is imaged on the basis of broadening of the spectrum due to an interaction of the measuring light with the organism, the artery and/or the vein can be imaged as in the preceding blood vessel imaging systems, where an optical heterodyne detection system is employed.
In all the blood vessel imaging systems of the present invention described above, broadening of the spectrum which corresponds to the flow rate of blood in the blood vessel detected when imaging the artery and/or the vein. Since the arterial blood and the venous blood are different from each other in the flow rate of blood, the artery and the vein can be distinguished from each other on the basis of broadening of the spectrum which corresponds to the flow rate of blood in the blood vessel. Further the flow rate of blood in the blood vessel can be known on the basis of broadening of the spectrum the similar manner, the flow rate of light scattering fluid other than blood can be also known.